Returning JSON from a JsonResult method in MVC controller

If the resultset string is already JSON (and not wrapped in any XML), then you'd want to return a ContentResult with exactly that string as the content:

public ContentResult GetCALMdata()
{
    CALMwsP.wsCALMSoapClient wsC = new CALMwsP.wsCALMSoapClient("wsCALMSoap");
    string resultset = wsC.GetRefTables("P_1", "P_2", "P_3", "P_4");

    return Content(resultset, "application/json");
}

You don't want to use JsonResult or the Json() helper in this case, because that's going to end up re-serializing your JSON.

If using ASP.NET MVC 2 or higher:

return Json(resultset, JsonRequestBehavior.AllowGet);